1. Field of the Invention
The present invention relates to a piezoelectric element having a multilayered structure (multilayered piezoelectric element) to be used as a piezoelectric actuator, ultrasonic transducer and so on, and a method of manufacturing the same.
2. Description of a Related Art
A piezoelectric material represented by a material having a lead-based perovskite structure such as PZT (Pb(lead) zirconate titanate) provides a piezoelectric effect of expanding and contracting when applied with an electric field. A piezoelectric element having the property is utilized in various uses such as piezoelectric pumps, piezoelectric actuators and ultrasonic transducers. The structure of a piezoelectric element is basically a single-layer structure in which electrodes are formed on both ends of one piezoelectric material. According to microfabrication and integration of piezoelectric elements with recent developments of MEMS (micro electro mechanical systems) related devices, multilayered piezoelectric elements each having plural piezoelectric materials and plural electrodes alternately stacked have been used. In such a piezoelectric element, the interelectrode capacitance of the multilayered structure as a whole can be made larger by connecting electrodes for applying electric fields to the respective plural piezoelectric material layers in parallel. Accordingly, the rise in electrical impedance can be suppressed even when the size of the piezoelectric element is made smaller.
As a related technology, Japanese Examined Patent Application Publication JP-B-61-32835 (Japanese Patent Application Publication JP-A-59-115579) discloses an electrostrictive effect element having films or thin plates of an electrostrictive material and internal electrode plates alternately stacked, in which end surfaces of the internal electrode plates are exposed on side end surfaces of the element and insulating layers are formed only on the exposed portions of the internal electrode plates and the electrostrictive material near the exposed portions on the side end surfaces (FIGS. 3 and 4).
In the electrostrictive effect element, the insulating film formed in a region including the end surface (side end surface) of the internal electrode is formed by depositing low-melting glass particles on the region by using electrophoresis, then heating the glass particles to a temperature higher than the glass softening point to melt them, and further, cooling and fixing them onto the region (page 3, FIG. 5).
In the case where the insulating film is formed according to the method, the following problems arise in the multilayered piezoelectric element. That is, since the glass material is relatively hard and has lower elasticity, when the piezoelectric material layer (the electrostrictive material film or thin plate in JP-B-61-32835) attempts to expand and contract when the multilayered piezoelectric element is driven, a brake is applied to the displacement of the region where the insulating film is fixed. Accordingly, it is hard to sufficiently bring out the performance of the piezoelectric material represented by electromechanical coupling factor k33 or the like. Here, an electromechanical coupling factor is a constant representing efficiency of converting given electric energy into mechanical energy (vibration energy) when a voltage is applied to electrodes provided on both sides of a piezoelectric material. Further, the electromechanical coupling factor k33 among the factors corresponds to an electromechanical coupling factor with respect to the same direction as a direction in which the voltage is applied, that is, an electromechanical coupling factor in the longitudinal vibration mode. The electromechanical coupling factor k33 is expressed by the following equation.k332=(mechanical energy utilized for vibration of the piezoelectric material)/(electrical energy applied to the piezoelectric material)
Especially, high transmission and reception sensitivity is necessary when the piezoelectric element is utilized as an ultrasonic transducer that transmits and receives ultrasonic waves in an ultrasonic diagnostic apparatus, and accordingly, it is desired that the performance of the piezoelectric material is maximized.
Further, in the conventional multilayered piezoelectric element, the glass particles are once melted for forming insulating films of glass, and the thickness of the insulating film becomes thinner and the width thereof becomes wider at the melting. Thereby, the withstand voltage between the internal electrode and the side electrode becomes lower, and the reliability of the multilayered piezoelectric element also becomes lower. Especially, in polarization treatment (poling) for providing a piezoelectric property to a piezoelectric element, a voltage larger than the drive voltage is applied, and breakdown occurs in a region having a low withstand voltage. Further, recent years, the piezoelectric material has been made thinner and thinner for downsizing of the multilayered piezoelectric element, and it is desirable that the width of the insulating film is as narrow as possible.